1. Field of the Invention
This invention relates to a toner used in electrophotograpy and electrostatic printing for developing an electrostatic latent image. The present invention also pertains to a two-component type, negatively chargeable developer containing a toner and a carrier.
2. Description of Prior Art
Methods for developing an electrostatic latent image into a toner image include a first method using a one-component type developer composed only of a toner and a second method using a two-component type developer composed of a toner and a carrier which are charged in opposite polarity.
The one-component type method may be, for example, a powder cloud developing method in which toner particles are sprayed over an electrostatic latent image-bearing surface, a contact developing method (touch down method) in which a latent image-bearing surface is directly contacted with a bed of toner, or a dielectric developing method in which a latent image-bearing surface is contacted with a magnetic, electrically conductive toner.
The two-component type method may be, for example, a magnet brush method using iron powder as a carrier, a cascade method in which glass beads are used as a carrier or a fur brush method in which a fiber brush is used as a carrier.
In the above developing systems, fine particles each composed of a matrix of a binder resin and a colorant, such as carbon black, dispersed in the matrix are generally used as a toner. To obtain clear images, a charge controlling agent is usually used. Known charge controlling agents include metal complexes of monoazo dyes; metal complexes of salicylic acid, naphthoic acid or a dicarboxylic acid; sulfonated copper phthalocyanine pigments; and nitrified or halogenated styrene oligomers; chlorinated paraffins; and melamine resins.
The known charge controlling agents suffer from one or more of the following defects and are not fully satisfactory. Thus, the conventional charge controlling agents are apt to be decomposed or deteriorated upon subjected to mechanical impact, heat or moisture to cause a change of the charging characteristics of the toner during use. In addition, a so-called filming phenomenon (formation of a film of the toner on surfaces of the photosensitive medium, blade and/or carrier) is caused upon repeated copying operations. Further, the conventional charge controlling agents are not easily uniformly distributed in respective toner particles to cause fogging of the copies.
In the two-component developer, relatively small particle size toner particles are electrostatically held on surfaces of relatively large particle size carrier particles. In development, the toner particles migrate to an electrostatic image against the attractive force of the carrier particles. Fresh toner particles are again held on the carrier particles by frictional electrification. During repeated use, however, a filming phenomenon is caused so that it is necessary to entirely replace the spent developer.
To minimize such filming of a toner on carrier particles, it is proposed to coat each carrier particle with a silicone resin. The silicone resin coat is, however, apt to be separated from the surfaces of the carrier upon subjected to mechanical shocks during use to cause a variation of charges of the developer. Additionally, a high electrical resistance of the silicone resin coat results in the local accumulation of charges so that the quality of developed images is deteriorated. To cope with this problem, it is proposed to incorporate an electrically conductive substance such as carbon or tin oxide into the silicone resin layer. This is not, however, effective to prevent the separation of the surface coating from the carrier. Further, the electrically conductive substance is apt to be separated from the coated layer.